1. Field of the Invention
The present invention relates to improvements in underfluid pelletizers and more particularly to providing a cutter hub position control device for an underfluid pelletizer including manual, incremental, and automated operation.
2. Description of the Related Art
Underwater pelletizers are well known and include a die plate with multiple orifices through which molten polymer or other melt-processable strands are extruded with the orifices terminating at a die face on the die plate. A powered rotary cutter including a cutter hub mounted on a motor shaft with a plurality of blades supported from the cutter hub is associated with the die face for shearing the extruded strands into pellets as the polymer is extruded beyond the die face. A transport fluid box encloses the die face, cutter hub and blades to form a cutting chamber through which transport fluid flows to quench and rigidify the extruded strands, thus enabling the cutting blades to better shear the extruded strands into pellets. A motor drives the pelletizer shaft through the transport fluid box and thus powers the rotary cutter. The above described pelletizers, specifically underwater pelletizers, are disclosed in related U.S. patents including U.S. Pat. Nos. 4,123,207, 4,251,198, 4,500,271, 4,728,276, 5,059,103, 6,332,765, and 7,033,152 all owned by the assignee of this invention.
Many known forms of underwater pelletizers rely on manual adjustment by an operator of the blades relative to the die face. This manual adjustment results in limited repeatability as not all operators make blade adjustments in the same way or at the same intervals. Manual adjustment is also affected by the fact that different operators will have varying levels of experience with the particular type of machine being adjusted. In addition, the operator may make too large an adjustment which is detrimental to the life of the blades. An adjustment that is too small could affect the properties of the end product.
Other forms of pelletizers use springs to hold the blades against the die face. As with manual adjustment, springs also produce inconsistencies in the amount of force put on the blades as the force is dependent on the degree to which the spring is compressed. Spring compression is affected by the amount of wear on the blades as well as the amount of wear on the die plate. Outside influences such as the fluid in the transport fluid box can also contribute to forcing of the blades against the die face, shortening the life of the blades.
Still other forms of known pelletizers have relied on a pneumatic cylinder to move the blades into the die plate. Due to the fact that the air in the cylinder is compressible, this methodology is also subject to the outside influences of the water. Substitution of the pneumatic cylinder with a hydraulic cylinder is possible, but the manner in which the adjustments are made can get quite complicated. Hydraulic systems also rely on expensive hydraulic pressure controlling components. Over time, these components may leak which can lead to unwanted movement of the pelletizer blades. At high pressures, the hydraulic lines can expand which would also allow for blade movement.
A hydraulic system also requires some type of feedback to let the control system know if the required blade adjustment action is taking place. This feedback can be provided by some type of load sensing device located at the die plate, or by the amperage (amp) load from the pelletizer motor. However, these feedback mechanisms may not always be accurate enough to properly relay what is occurring.
More specifically, the majority of underwater pelletizer applications require that the blades be at an angle in relation to the die plate in order to cut the pellet and move it away from the die as quickly as possible. Having the blades at an angle and mounted at a given radial distance away from the pelletizer shaft creates a condition in which the action of the blades in the water tends to push the blades against the die plate, much like a boat propeller moves a boat. With either of the load sensing mechanisms identified above, it is possible to get false readings simply due to the action of the fluid.
In addition, certain polymers being extruded and cut by the pelletizer have lubricating qualities. If the amp load of the pelletizer motor is used to detect the load at the die plate, an amp increase may not be generated if the blades cannot create friction on the die plate due to the lubricity of the polymer. This absence of amp increase will result in more force than what is necessary to create the desired load, which is detrimental to the life of the blades.
In sum, the number of blades on the cutter hub, the width of the blades, the transport fluid flow rate, the material being processed, the material flow rate, and the pelletizer speed can all contribute to additional loads on the pelletizer motor. If any of these factors is changed, a different motor load increase will likely be obtained, making blade adjustments difficult, if not impossible, to repeat.
U.S. Pat. No. 3,832,114 discloses a manual coarse and fine adjustment mechanism used independently to optimize the positioning of the cutter hub and thus the cutter blades against the die face. U.S. Pat. No. 3,912,434 automates the coarse and fine adjustment wherein the coarse adjustment is achieved through use of a compressed air cylinder and fine adjustment is accomplished utilizing an electrically controlled worm gear assembly. Feedback is achieved through use of vibrational and/or electrical sensors insulatingly embedded in the die face on the surface in contact with the blades. Limits are determined by the magnitude of the electrical impulse generated.
Use of a stepper motor to control motion of a pelletizer is disclosed is U.S. Pat. No. 4,529,370 wherein the stepper motor controls a gear that interfaces with a first main piston to move the cutter hub in small increments. The gear positions a stopper flange that resists the action of a second piston to pull the main piston with the attached cutter hub toward the die face. As the stepper motor rotates incrementally, the second piston is allowed to draw the main piston to better engage the cutter blades with the die face. Conductivity between the cutter blades and the die face, more specifically, a capacitance bridge circuit, serves as the feedback mechanism to facilitate automation of the stepper motor adjustment.
Control of the movement of the cutter hub and blades as disclosed in U.S. Pat. No. 5,330,340 (“the '340 patent”) is achieved by positioning a threaded cylinder containing a shaft through the combination of an incremental drive motor having a gear mechanism oriented transaxially to a second coaxial shaft with gears that intermesh with a main gear circumferentially about the threaded cylinder and to which is attached a compatibly threaded drive thus engaging with and moving the cylinder forward or rearward to adjust the cutter hub and cutter blades against the die face. The '340 patent includes a feedback mechanism that relies on vibration of the cutter blades against the die face, as monitored by a piezoelectric accelerometer. Specific frequencies are identified and subsequently used to monitor proximity of the blades to the die face and adjust accordingly. The cutter hub mechanism as disclosed in the '340 patent works from the upstream or polymer feed side as opposed to similar mechanisms described hereinabove as disclosed in U.S. Pat. No. 4,529,370.
Indexing of the pelletizer utilizing a servo motor is disclosed in U.S. Pat. No. 6,217,802 (“the '802 patent”). The device as disclosed in the '802 patent automatically advances the pelletizer knives a predetermined distance toward the die face on expiration of a predetermined time period. Preferably a set number of equivalent distance advancements are made automatically at those fixed intervals determining the life of the knives. The pelletizer knives are initially calibrated by advancing them toward and against the die face such that the amps measuring the load or resistance generated on the drive motor is within a predetermined range. From this calibration, the number of incremental advancements is determined.
Blade adjustment using a servo motor is also disclosed in U.S. Pat. No. 6,663,372 (“the '372 patent”) which monitors the force on the blades in order to make the blade adjustments, ultimately keeping a given amount of force on the blades. Different materials act differently and some require more force than others such that the unit disclosed in the '372 patent would have to be set up for each individual polymer. Adjustment of the position of the cutter hub and cutter blades in relation to the die face is accomplished by controlling the motion of a carriage on which the pelletizer motor is mounted relative to the support frame.
In view of the foregoing, a need exists for a means of adjusting the blades in an underfluid pelletizer that produces highly repeatable results, the adjustment of which can be made manually, at set time intervals, as well as automatically, such that the cutter hub position control device is capable of accommodating and compensating for outside influences on the blades.